skeleto don't damage with damage potion

and crash
---- Minecraft Crash Report ----
// Quite honestly, I wouldn't worry myself about that.

Time: 06.08.14 14:26
Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$KeyIterator.next(Unknown Source)
at qi.a(SourceFile:163)
at net.minecraft.server.MinecraftServer.y(SourceFile:614)
at net.minecraft.server.MinecraftServer.x(SourceFile:530)
at cxc.x(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:446)
at java.lang.Thread.run(Unknown Source)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– System Details –
Details:
Minecraft Version: 14w31a
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_65, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 462641808 bytes (441 MB) / 722886656 bytes (689 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qr['robert14'/124, l='Nowy świat', x=246,20, y=67,87, z=155,45]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.